Fusion causes confusion

Fusion beats are an amalgam of two competing rhythms within the same chamber, atrial or ventricular. They are not “abnormal” beats and are a lesson in timing. 

For fusion beats to occur, two foci must commence propagation at or near the same time and both contribute to chamber depolarization. Both must be either atrial or ventricular and depending on the contribution of each, the resultant progeny have similarities to one or both parents. 

Atrial fusion beats occur as a result of competition between two atrial sites, usually sinus with ectopic atrial rhythm or atrial pacing. Only P waves show fusion.

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Ventricular fusion beats occur as a result of competition between two foci: usually one is sinus generated together with an ectopic ventricular focus or ventricular pacing. Only QRS complexes show fusion.

Late ventricular or “end-diastolic” ectopics, which occur in the PR interval of the next sinus cycle may fuse with the next sinus generated QRS producing a ventricular fusion beat.

Ventricular fusion is also seen with intermittent complete AV block and competition with the ectopic ventricular escape focus when both are similar rates. 

Ventricular fusion can also occur with the Wolff-Parkinson-White (WPW) ECG pattern of pre-excitation. Whereas the accessory pathway usually conducts to the ventricle faster than AV conduction resulting in a short PR interval, delta wave and broad QRS, on occasion AV conduction also occurs resulting in ventricular fusion. This results in a modified appearance to the WPW pattern.  

Ventricular fusion is frequently seen with ventricular parasystole and aids in the diagnosis.

Remind me about parasystole!

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A parasystolic focus is an ectopic focus that competes with a primary (dominant) focus, so there are two intrinsic pacemakers each having different cycle lengths. It can occur at all levels in the heart and there is an entrance block protecting the secondary parasystolic focus, so that impulses cannot enter and reset the timing of the parasystolic focus. The parasystolic block is therefore unidirectional and whenever the parasystolic focus depolarizes, it will conduct to the surrounding myocardium, provided it is not refractory. Think of a pacemaker, programmed asynchronous (AOO, VOO) at a rate of about 40 bpm in competition with the dominant rhythm. The reported repetition rates for parasystole range from about 15 to 70 bpm with the slower rates being in the ventricle. On the ECG, at least three parasystolic complexes are required for the diagnosis, and long rhythm strips may be required.

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Ventricular parasystole is the most easily and commonly recognised parasystole and usually occurs with sinus rhythm as the dominant rhythm. It masquerades as ventricular ectopy such as trigeminy or quadrigeminy but differs in that there are varying coupling intervals between sinus beats and the ventricular complexes. Because of an entrance block, the dominant rhythm cannot enter the parasystolic focus and reset its timing. Whenever the parasystolic focus depolarizes, it will conduct to the surrounding myocardium, provided the tissue is not refractory.

• Sinus rhythm 75 bpm.
• Unifocal, parasystolic ventricular ectopics (blue arrows, red highlight) with a fixed inter-ectopic interval of 3640 ms (17 bpm).
• The ventricular parasystolic focus is not reset by the sinus generated QRS complexes and thus there is a variable sinus generated QRS to ventricular ectopic coupling interval (600 ms to 660 ms).
• Ventricular fusion (purple highlight, blue stippled arrow) results when a scheduled ventricular parasystolic beat falls within the PR interval (end diastole) of the sinus complex. 

Another example (purple highlight):

When the ectopic timing occurs during the ventricular refractory period, the parasystolic output is inhibited (green highlight).

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By far the most common cause of ventricular fusion beats is with atrial fibrillation and ventricular pacing. There are usually frequent pauses with the native rhythm allowing ventricular pacing. At the end of the pause, a native ventricular complex may compete with ventricular pacing resulting in a fusion beat (purple highlight).

With dual chamber pacing, the PR interval of the native rhythm (As Vs) may be near identical to the programmed AV delay so that a sinus generated native ventricular wave of depolarization fuses with the paced QRS for long periods. The ventricular fusion is usually not recognized on the ECG and can be prevented by ventricular pacing minimization algorithms (section 16.17). They can also be recognized on Holter monitoring when “forced” ventricular pacing occurs during auto-threshold testing algorithms (section 16.18).

When ventricular fusion occurs with biventricular pacing, there are now three ventricular waves of depolarization involved in the fused complex.  

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Harry Mond